This invention relates to an apparatus for the generation and recycling of a sulfur trioxide reagent in a carrier, and, more particularly, to a system incorporating such sulfur trioxide reagent generating apparatus for the surface treatment and sulfonation of polymeric materials.
Sulfonation of polymeric resins, that is, the introduction of the --SO.sub.3.sup.- or --SO.sub.3 H functional group onto the surface of polymeric materials is generally known. See, for example, Walles patents No. 2,832,696; 2,937,066; 3,592,724; 3,613,957; 3,625,751; 3,629,025; 3,770,706; 3,959,561; 4,220,739; and 4,615,914. Typically, the sulfonation is carried out by using gaseous mixtures of dry air containing from 2 to 8% sulfur trioxide which are then reacted with the polymeric material. Several known systems may be used to produce the sulfur trioxide. For example, oleum (concentrated sulfuric acid containing sulfur trioxide) (H.sub.2 S.sub.3 O.sub.10) has been used as a source of sulfur trioxide gas. In such a system, dry air is passed through the oleum to facilitate sulfur trioxide stripping of the oleum by mass transfer.
Cameron et al, U.S. Pat. No. 4,663,142 discloses a continuous process for the generation of sulfur trioxide from oleum which introduces oleum feed to a sulfur trioxide desorption tower to form a gaseous mixture of dry air and sulfur trioxide. Masse et al, U.S. Pat. No. 4,673,560 teaches a process and apparatus for the generation of sulfur trioxide using microwave energy. A sulfur trioxide-rich oleum feed is subjected to microwave energy for a time sufficient to produce a sulfur trioxide vapor which is then mixed with dry air. In both processes, large amounts of spent acid are produced which must be disposed of or recycled in some manner. See also, Walles U.S. Pat. No. 4,615,914, which teaches conversion of solid pills of polymeric sulfur trioxide into an air-sulfur trioxide gas mixture via microwave energy. This process leaves no residue.
One use of sulfur trioxide has been the surface treatment of a variety of polymeric resins to chemically modify their surfaces by a sulfonation reaction. For example, such surface sulfonated polymers are useful as substrates for painting and metal coating and are also useful as enclosure members for containing hydrocarbons such as gasoline and the like. Exemplary uses include containers such as gasoline and other fuel tanks, fuel barrels and drums, oleaginous food containers such as bags, tubs and cartons; fibrous materials for use in carpets, clothing and other fabric; plastic substrates and metal-clad plastics such as capacitors, auto parts and the like; and plastic substrates for use in electrostatic spray painting and the like.
Likewise, various medical devices are fabricated of or contain a variety of polymeric resins such as polycarbonates, polyurethanes, polysiloxanes and polyolefins. These polymeric resins are used to form housings, tubes, valves, and the like. Many of these medical devices are designed to come into contact with blood or other body fluids, either during removal from the body, during treatment of the fluid, or during the return of the fluid to the body. For example, such devices may include blood filters, blood oxygenerators, dialyzers, tubing and the like. One basic requirement for all such medical devices is that the surfaces which contact the blood or other body fluid of a patient be water wettable.
Wettability is needed to prevent air bubbles from sticking to a surface and ending up in a patient's blood, or causing irregular flow through a tube or the like. Wettability is also important for preventing blood from sticking to or coagulating on a surface. However, most, if not all, of the plastic resins utilized in such medical devices have hydrophobic surface properties. Sulfonation of such surfaces becomes necessary to modify the surface properties of such resins to make those surfaces hydrophilic.
However, because such devices are to be used in medical applications and are designed to come into contact with body fluids, the sulfonation reaction must be controlled carefully. The strength of the sulfur trioxide reagent must be maintained within strict limits. If the strength of the reagent varies during treatment, the surfaces of the devices may be inadequately sulfonated necessitating the discarding of such devices. Additionally, the presence of even trace amounts of water may cause the formation of sulfuric acid which may adhere as small droplets to the surfaces of the plastic to be treated and cause local irregularities. Finally, the generation of sulfur trioxide reagent as well as treatment of the surfaces of these products requires large volumes of the highly dilute sulfur trioxide reagent to be passed through the system. This results in large volumes of acid waste which must be properly disposed of or recycled in some manner.
Accordingly, there remains a need in the art for a system for generating sulfur trioxide in controllable concentrations, and with an absolute minimum of impurities for the surface treatment and sulfonation of polymeric resin materials, particularly those used in medical devices. Further, there remains a need for a system which minimizes the amount of waste acid which requires disposal.